Hi there!
Great question!
Fewer kinds of organisms live in deserts than forests because they have special adaptions to their own home... and there are a few, loooong examples below for you to read.
ENJOY!
Camels have a water storage in their humps on their back, so they can go a long time without drinking any water. They have tan fur color to blend in with the sand for predators. They have a short-haired coat on most of their body because of the hot sun, but because deserts can get very cold at night (almost dropping temps below 0), they have thicker fur on their humps and neck. If they were moved to the forest biome, they would take a long time to adapt to their new home.
Next example can be a wolf. A wolf can live in the forest because it has adapted over time to make a wolf, well, a wolf. It has thick fur for keeping it warm at night and in wintertime. A wolf has such a particular diet, that if it lived in a desert, it wouldn't find much food. In the desert, there are snakes and scorpions and bugs, whereas the forest has the deer and caribou to eat.
See, so its pretty simple. Basically, animals live where they live for a reason. If you just moved them to somewhere else in the world, and this could be anywhere, they would take a loooong time to adapt to their new homes.
* A great example of animal adaptation over time is the spirit bear. A spirit bear is a black bear, with white fur, and they aren't albinos. Its a hair color that is very rare, and according to scientific studies, the ratio to spirit bears and black bears in the forest is 1:10. They have developed white fur overtime because the black bears are much darker than the spirit bears, and fish underwater, their source of food, can see them right above the water before the bear dives for a meal. The Spirit bear, however, has a better time catching the fish underwater because it has white fur, not allowing the fish to see her above. Then she dives in for the catch and is successful! *
Hope this helps!
Hope you learned something new! :D